Got my sequencing a bit wrong - what ASHP pipe sizing to allow for in new slab?

[low_and_there]

Looking for a bit of help: We're about to install an insulated raft foundation (300mm of EPS with a 100mm concrete slab on top) and have planned for all the pipework under the insulation excepting the future route for the ASHP pipes and electricity supply to it...

 

I was planning on putting in a route for pipes to be inserted through later on when the ASHP gets installed but have failed to get very far with an installer - is there a reasonably safe size of ducting I could use to enable future insulated pipes from the outside ASHP to be routed through under the slab and into the thermal envelope of the house? For the electrics I am just going to put in a big pipe with rodding access for cabling to be pulled through in the future.

 

Am self-managing our renovation and like most renos it's not gone to plan or budget and all my planned dependencies have been turned upside down.

 

thanks in advance

[IanR]

What size ASHP?

 

You'll be safe if you go with Ø32 pipe, but maybe you could go smaller.

 

If you plan to run these in through a duct later though then how are you going to insulate them?

 

You should get in something like Rauvitherm pre-insulated twin duct from Rehau. The 32/150 duo is pretty well insulated, but needs a deep trench due to the minimum bend radii. Most SE specify that the pipes/ducting must come up vertically through the raft. iirc the 32/150 Duo needs a 750mm deep trench (below insulation) in order to get it coming vertical through the raft.

 

Edited December 13, 2023 by IanR

[JohnMo]

And how far are you running the pipe, a metre or many metres all make a difference.

 

Insulation is important depending on where the heat may be escaping too. 60 Deg water going to heat your cylinder needs insulation.

 

Is under the floor the only way from A to B?

[low_and_there]

27 minutes ago, JohnMo said:

And how far are you running the pipe, a metre or many metres all make a difference.

 

Insulation is important depending on where the heat may be escaping too. 60 Deg water going to heat your cylinder needs insulation.

 

Is under the floor the only way from A to B?

 

@JohnMo under the floor isn't the only way, no - I had at some point got to that conclusion but now you ask I am actually not 100% sure that I recall why that was. It could just as well go through the external wall and in.

 

Given what @IanR has pointed out about needing a very deep trench in order to come up vertically, perhaps that is altogether better...?

 

To your questions JohnMo, the outside unit is going to be placed 3m from the external wall but in total about 9m (as the crow flies) to the hot water cylinder. If it comes in through the external wall it would probably need 5m outside and then about 3m along the wall inside.

[Dan F]

We used Rauvitherm pre-insulated twin duct and put this in before the hardcore below EPS.  You need a deep trench, but it's not that bad if you put this in before everything else. This goes through hardcore, EPS and concrete without any ducting.

 

Our M&E consultant told us to use 25+25mm but that was bad advice as 25mm plastic isn't much more than 20mm.   Our 7kW AroTherm Plus manages 1200L/h to our buffer, but I'm pretty sure there would be no remaining head for much else.  Length is 8m or so, if it had been any longer it would have been worse.  5kW ASHP would have been less of an issue with this size, but most people would still use 28mm copper for 5kW.

 

Do you know what your approx heat loss is going to be. This will help, because it house is huge and heatless is 10=kW, then you might need more than 32m, especially if plastic with smaller ID.

Edited December 13, 2023 by Dan F

[JohnMo]

11 minutes ago, low_and_there said:

Given what @IanR has pointed out about needing a very deep trench in order to come up vertically, perhaps that is altogether better...?

 

If you go 32/150 Rauvitherm pre-insulated twin duct, your bend radius is around 1m plus, super stiff, so not really practical to install later. You can add bends but not sure how in concrete casting.

 

I would go over ground, holes are easy enough through wall, pipe easy to insulate etc.

[JohnMo]

9 minutes ago, Dan F said:

Our M&E consultant told us to use 25+25mm but that was bad advice as 25mm plastic isn't much more than 20mm.   Our 7kW AroTherm Plus manages 1200L/h to our buffer, but I'm pretty sure there would be no remaining head for much else

Nothing wrong with that size, head loss over 100m is around 12m (for 22mm Hep2O which is smaller again), so over 8m (16m total), would loose less than 1.2m head against 6 to 8m pump. So he/she was correct and plenty of room to loads of other thing and for the circulation pump to modulate.

 

 

[ReedRichards]

1 hour ago, low_and_there said:

is there a reasonably safe size of ducting I could use to enable future insulated pipes from the outside ASHP to be routed through under the slab and into the thermal envelope of the house?

Maybe I am reading this wrong but I would have thought that the sooner you get your pipes inside the thermal envelope of the house the better.  No matter how well insulated, those pipes will lose some heat and if they do this inside the thermal envelope then it's not wasted heat.

[HughF]

15m of 32+32 from ingoodnic here on my install, 9kW unit on the end… It was like wrestling a python.

[Dan F]

1 hour ago, JohnMo said:

Nothing wrong with that size, head loss over 100m is around 12m (for 22mm Hep2O which is smaller again), so over 8m (16m total), would loose less than 1.2m head against 6 to 8m pump. So he/she was correct and plenty of room to loads of other thing and for the circulation pump to modulate.

 

Maybe, but pump runs at 100% to achieve 1200L/hr so I'm not sure there would be any remaining head for UFH if I was running an open-circuit with no buffer!  Also Vaillant people not happy really (not that that has been an issue in practice).

Edited December 13, 2023 by Dan F

[jack]

On 13/12/2023 at 18:43, low_and_there said:

Looking for a bit of help: We're about to install an insulated raft foundation (300mm of EPS with a 100mm concrete slab on top) and have planned for all the pipework under the insulation excepting the future route for the ASHP pipes and electricity supply to it...

 

300 mm is usually made up of 3 x 100 mm layers. We have similar, and I ran some cheap drainage conduit through two channels I cut into the middle layer of insulation when the guys were onsite doing the foundations. I was able to bend a reasonable radius up into the plant room. No issue pulling the flow and return pipes through later.

 

More detail here (worth reading both threads in their entirety, but particularly the first one):

 

[haythorn_1]

22 hours ago, HughF said:

15m of 32+32 from ingoodnic here on my install, 9kW unit on the end… It was like wrestling a python.

This looks much cheaper than the Rauvitherm, is there much difference?

[IanR]

2 hours ago, haythorn_1 said:

This looks much cheaper than the Rauvitherm, is there much difference?

 

It's just a smaller OD, so 20mm less insulation all round. The equivalent Rehau product to that is the Rauthermex 32mm+32mm/111m